The present invention relates to a biaxially-oriented polyolefin film which can be sealed on both sides and is comprised of three layers, the base layer consisting essentially of propylene polymers and the two sealing layers consisting essentially of sealable olefin polymers, and which is distinguished by universal applicability to high-speed packaging machines and is also readily printable. The present invention also relates to a process for the preparation of the film and to its use.
EP-A-0 008 904 discloses a biaxially-oriented, three-layer polyolefin film which can be sealed on both sides and in which the base layer is formed by propylene polymers and the two sealing layers, i.e., the outer or top layers, are formed by sealable olefin polymers. Although this polyolefin film has good heat-sealability, it has, in particular, only a low scratch resistance, cannot be printed and also leaves much to be desired with respect to transparency and slip properties in conjunction with high-speed packaging machines.
German Offenlegungsschrift 3,247,998 discloses a biaxially-oriented polyolefin film consisting of three layers which is transparent and particularly readily sealable. Its base layer consists essentially of a propylene polymer, and the two sealing layers consist of an appropriate olefin polymer containing, as additives, 5 to 15% by weight of a low-molecular resin which is compatible with the olefin polymer, 5 to 15% by weight of a propylene homopolymer and 0.3 to 1.5% by weight of polydiorganosiloxane, the percentages by weight relating in each case to the sealing layer. This polyolefin multi-layer film possesses characteristics which are important for packaging films, namely a wide range of sealing, a low sealing temperature, high gloss and good transparency. The film also displays a relatively high scratch resistance and low friction and hence good running characteristics on high-speed packaging machines of various types. However, it lacks another characteristic which is also important, namely good printability.
A biaxially-oriented polyolefin film which can be sealed on both sides and comprises three layers is also disclosed in U.S. Pat. No. 4,419,411. In this film, an additive combination is also incorporated in both of the sealing layers. They each contain 0.15 to 0.4% by weight of polysiloxane and 0.05 to 0.5% by weight of silicon dioxide, the percentages by weight relating in each case to the sealable layer. The base layer is made of polypropylene (as the main component) and contains a small amount of a monocarboxamide. Part of the amide migrates from the base layer into the two sealing layers and onto the surfaces thereof (external faces), so that polysiloxane, silicon dioxide and the monocarboxamide are present in incorporated form in each sealing layer, and the monocarboxamide is also present on the outer faces of the two sealing layers. The polyolefin multi-layer film described is stated to have a particularly low coefficient of friction. However, this film also suffers in particular from the disadvantage that it is not printable.